VFD question

[Glenn Brooks]

Hi all, my old iron (2/5 hp 220 volt 3 phase 12" x 48") Standard Modern gear head lathe has a growler low end- lowest speeds are 65, 100, and 150 RPM. These have seen hard military use since 1958 and while still functional, are pretty noisy. The gears are physically worn and will be candidates for replacement at some point.

The higher end in contrast spins like new: 400, 600, 1250 RPM sounds and looks great.

Aside from reading various threads, I have no experience with VFD's.

So, would it be practical to install a VFD to run the machine in one of the higher gears and use a VFD to operate the machine in the 50-250 RPM range? Thus extending the life of the gear box for possibly many years into the (my) future?

If so, would the lathe have equivalent torque etc when making heavy cuts in low gear?

Thanks much for advice you might be able to offer.

Glenn B.

[Dave_C]

Glenn,

I would say yes! Using the 400 rpm gear and running your drive down to 20% you would have 80 RPM. Some go as low as 15% but that depends on the motor you choose.

Is 2/5 hp a typo or is that what you have? I'd go for at least 1 hp or more, maybe 2 hp or 3 as the cost is actually cheaper than the 1 hp motor. That way you have some mass to keep spindle speed tracking and some reserve torque.

Modern VFD's are pretty easy to set up. Most have a "quick setup" set of instructions that come with the instruction book.

If you already have 3 phase then you're half way home anyway! If not, get a 2hp setup that runs on 240 1 phase and pust out 240 3 phase. That is what I have and it works great! I keep my gears set at 1,000 RPM and rarely change them. I have higher and lower but I rarely need to make a change.

If you need help just ask and we'll see what kind of answers we can come up with.

Good luck.,

Dave C.

Edited spelling 10-13-16

[SteveHGraham]

Oddly, sometimes you can get big motors cheaper than small ones. Ebay is full of motor bargains.

[Glenn Brooks]

Thanks Dave C and Steve,

Yes, the motor is a dual horsepower 2hp and 5 hp motor. I think the low rating drives one set of gears and the 5 hp drives the high end gears.

I run the lathe on an existing 10 hp RPC. So it has three phase power into the machine.

From what you say, it sounds like a VFD is a very doable solution.

Much appreciate the advice!

Glenn

[wlw-19958]

Hi There,

Yes, the motor is a dual horsepower 2hp and 5 hp motor. I think the low rating drives one set of gears and the 5 hp drives the high end gears.

That is unusual from my experience. From what I have seen,
motors with dual HP ratings are motors with dual RPM ratings.
Is this what you meant?

Good Luck!
-Blue Chips-
Webb

[GlennW]

My lathe is 3/6 hp and works as Webb states.

You can run the motor on high or low for each headstock setting giving two different spindle speeds for each setting.

I know nothing about VFD's but have always been under the impression that you lose considerable torque with decreased motor rpm.

Is this not true?

[Dave_C]

Glen,

It is not as big an issue as it once was. The old VFD's did not pull worth a hoot at low speed!

Modern VFDs actually have two choices when you set them up. Constant torque is the one you want for a lathe.

Modern motors that are rated for VFD use do a great job. But a side note: If you plan on running a motor at low speed a lot, get a motor that has the AUX fan built onto the back of the motor. It will keep the motor cool even though it is being used at a slow speed. The motors without this fan don't turn fast enough at low speed to throw enough air with the centrifugal fan that is attached on the end.

VFDs, like a lot of other things, have come a long way since I first started working with them back in the 80's.

Of course you can stall any motor if you overload it. I use some of my lower gears if I need full HP and torque at low speeds. That is one of the nice things about having 9 gears to chose from. I would never go back to my fixed speed motor! (not to mention the fact that the 3 phase motor makes better cuts)

Dave C.

[GlennW]

Thanks Dave!

[Glenn Brooks]

Hi There,

Yes, the motor is a dual horsepower 2hp and 5 hp motor. I think the low rating drives one set of gears and the 5 hp drives the high end gears.

That is unusual from my experience. From what I have seen,
motors with dual HP ratings are motors with dual RPM ratings.
Is this what you meant?

Good Luck!
-Blue Chips-
Webb

Hi Webb, yes maybe... well actually I don't understand exactly what my motor actually does with the two HP ratings. So your description is probably accurate. Just know the manual describes a 2/5 configuration and shows some wiring diagrams. But there's no understandable description, except something like " turn on the on switch to make chips..." being slightly electrically challenged, I've been happy to stay at that level...

Actually what I've gleaned from everyone's comments so far is my old motor may possibly overheat if operated to long at low RPMs. I am sure this motor was not designed to accommodate VFD operation, as it was built long before VFD's were a gleam in anyone's eyes. Wondering if this might be a deal killer?

Glenn B.

[Dave_C]

Glenn, B.

I think I missed that part about keeping your existing motor. Funny how we assume things and then we turn out to be wrong.

The old motor was not designed for a VFD but it may do very well as most of the old motors were wound better than our modern ones and probably have more mass to them.

It is not hard to install a small box fan on the end of the motor that runs at full speed all the time for cooling. Way cheaper than buying a new motor!

Questions: Does your motor have cooling fins and any sort of fan on the end of it now or is it an open frame design?

Dave C.

[Glenn Brooks]

Dave, I will have to go look at in the morning. I think it is a sealed cause with no significant cooling fins that I recall, also definitely no exterior fan. There might be a small, narrow shielded opening around the case. But I haven't looked at it closely for a long while so memory is fuzzy. I'll post a picture or two here tomorrow. Funny thing is there is no manufacturers plate on the motor . Or it was a decal that has come off and/or been painted over long ago. (It's a Canadian Navy lathe originally). So a paucity of background info to work with...

[Dave_C]

Glenn,

Sounds good. I may not be online or I may if I have access to wi-fi where I am going this week. So I'll get back to you as soon as I have access to the web.

(Taking the Motor home to Ft. Gibson Lake for 10 days and then maybe to Houston to visit NASA and the battleship Texas if the weather holds)

Sooner or later "I'll be back" [Arnold S]

Davc C.